Optical modulators are known in the art. In many applications, in particular for high speed optical communication systems, a modulated light wave is used to carry digital information from a sender to a receiver. In many such systems, the modulation may be phase and/or amplitude modulation. Examples include binary amplitude modulation with a return-to-zero (RZ) or non-return-to-zero (NRZ) optical pulse stream format, and phase shift keying modulation techniques, such as Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), and Quadrature Amplitude Modulation (QAM) techniques, such as QAM8, QAM16 and QAM64. In each of these communication formats, the modulated light wave will carry information about one or several symbols selected among a predetermined set of symbols.
In order to achieve such modulation of a carrier light wave, it is known to split the carrier light wave using a splitter, and to recombine the carrier light wave in a combiner after a relative phase shift of the different light paths between the splitter and combiner, forming a Mach-Zehnder interferometer. The phase shift can for instance be achieved using electrodes attached to one or more of said paths, to each of which electrodes a variable electric signal can be applied so that the refractive index of the path waveguide material changes. Such variable phase shift can be combined with a predetermined fixed phase shift for each waveguide. This way, each symbol can be modulated as a unique combination of total phase shifts along each path.
Some modulators can comprise a plurality of so-called “child” interference modulators that are arranged in parallel, and that are comprised within a larger “parent” interference modulator. A modulator in which a first parallel-coupled MZM controls the imaginary part of the electromagnetic field (Q value) and a second parallel-coupled MZM controls the corresponding real part (I value) is called an IQ modulator (IQM).
WO 2011022308 A2 discloses using a Mach-Zehnder modulator (MZM), yielding two paths, or two parallel-coupled child MZMs each on one respective path of a parent MZM, yielding in total four paths, with variable-voltage electrodes on each path, for such modulation.
A problem with such interferometric light wave phase modulators is that these modulators are typically very large when compared with the wavelength of the light in the optical waveguides. This is attributable not only to the typically weak refractive index modulation response of the waveguide material in response to applied voltage, but also to the additional device length that is required to accommodate the optical splitters and combiners and their input and output access waveguides. The relatively large device dimensions have an impact on the cost of the modulator device itself, and the chip size will also impact the size and cost of the modulator submount and of the optical module package enclosure.
Another problem associated with the typically weak refractive index response of the waveguide material to reverse voltage bias is that either lengthy phase modulation electrodes are required, and/or high modulation voltage swings are required, to attain a desired dynamic phase modulation amplitude or intensity extinction ratio under device operation. A problem with lengthy electrodes on the phase modulator arms is that these contribute a parasitic capacitance that limits the maximum achievable modulation bandwidth of the modulator. The use of travelling wave electrodes partly mitigates the bandwidth limitation that is imposed by the capacitance on the arms, but these travelling wave electrodes are typically longer, and they are more complex to manufacture, than the basic “lumped” electrical circuit element phase modulation electrodes.
An additional problem with the use of large amplitude dynamic voltage swings is that the electrical power dissipation is approximately proportional to the square of the modulation voltage swing amplitude, so that a large voltage swing will contribute substantially to the energy cost of the operation of the communication system.